HDAC2 Regulates Site-Specific Acetylation of MDM2 and Its Ubiquitination Signaling in Tumor Suppression

Histone deacetylases (HDACs) are promising targets for cancer therapy, although their individual actions remain incompletely understood. Here, we identify a role for HDAC2 in the regulation of MDM2 acetylation at previously uncharacterized lysines. Upon inactivation of HDAC2, this acetylation creates a structural signal in the lysine-rich domain of MDM2 to prevent the recognition and degradation of its downstream substrate, MCL-1 ubiquitin ligase E3 (MULE). This mechanism further reveals a therapeutic connection between the MULE ubiquitin ligase function and tumor suppression. Specifically, we show that HDAC inhibitor treatment promotes the accumulation of MULE, which diminishes the t(X; 18) translocation-associated synovial sarcomagenesis by directly targeting the fusion product SS18-SSX for degradation. These results uncover a new HDAC2-dependent pathway that integrates reversible acetylation signaling to the anticancer ubiquitin response.

Skp2-dependent reactivation of AKT drives resistance to PI3K inhibitors

The PI3K-AKT kinase signaling pathway is frequently deregulated in human cancers, particularly breast cancer, where amplification and somatic mutations of PIK3CA occur with high frequency in patients. Numerous small-molecule inhibitors targeting both PI3K and AKT are under clinical evaluation, but dose-limiting toxicities and the emergence of resistance limit therapeutic efficacy. Various resistance mechanisms to PI3K inhibitors have been identified, including de novo mutations, feedback activation of AKT, or cross-talk pathways. We found a previously unknown resistance mechanism to PI3K pathway inhibition that results in AKT rebound activation. In a subset of triple-negative breast cancer cell lines, treatment with a PI3K inhibitor or depletion of PIK3CA expression ultimately promoted AKT reactivation in a manner dependent on the E3 ubiquitin ligase Skp2, the kinases IGF-1R (insulin-like growth factor 1 receptor) and PDK-1 (phosphoinositide-dependent kinase-1), and the cell growth and metabolism-regulating complex mTORC2 (mechanistic target of rapamycin complex 2), but was independent of PI3K activity or PIP₃ production. Resistance to PI3K inhibitors correlated with the increased abundance of Skp2, ubiquitylation of AKT, cell proliferation in culture, and xenograft tumor growth in mice. These findings reveal a ubiquitin signaling feedback mechanism by which PI3K inhibitor resistance may emerge in aggressive breast cancer cells.

Acetylation-dependent regulation of MDM2 E3 ligase activity dictates its oncogenic function

Abnormal activation of the oncogenic E3 ubiquitin ligase murine double minute 2 (MDM2) is frequently observed in human cancers. By ubiquitinating the tumor suppressor p53 protein, which leads to its proteasome-mediated destruction, MDM2 limits the tumor-suppressing activity of p53. On the other hand, by ubiquitinating itself, MDM2 targets itself for destruction and promotes the p53 tumor suppressor pathway, a process that can be antagonized by the deubiquitinase herpesvirus-associated ubiquitin-specific protease (HAUSP). We investigated the regulation of MDM2 substrate specificity and found that acetyltransferase p300-mediated acetylation and stabilization of MDM2 are molecular switches that block self-ubiquitination, thereby shifting its E3 ligase activity toward p53. In vitro and in cancer cell lines, p300-mediated acetylation of MDM2 on Lys¹⁸² and Lys¹⁸⁵ enabled HAUSP to bind, presumably deubiquitinate, and stabilize MDM2. This acetylation within the nuclear localization signal domain decreased its interaction with the acidic domain, subsequently increased the interaction between the acidic domain and RING domain in MDM2, enabled the binding of HAUSP to the acidic domain in MDM2, and shifted MDM2 activity from autoubiquitination to p53 ubiquitination. However, upon genotoxic stress through exposure to etoposide, the deacetylase sirtuin 1 (SIRT1) deacetylated MDM2 at Lys¹⁸² and Lys¹⁸⁵, thereby promoting self-ubiquitination and less ubiquitination and subsequent degradation of p53, thus increasing p53-dependent apoptosis. Therefore, this study indicates that dynamic acetylation is a molecular switch in the regulation of MDM2 substrate specificity, revealing further insight into the posttranslational regulation of the MDM2/p53 cell survival axis.

Diminished DYRK2 sensitizes hormone receptor-positive breast cancer to everolimus by the escape from degrading mTOR

Mammalian target of rapamycin (mTOR) inhibitor, everolimus, provides benefit for metastatic hormone receptor positive breast cancer after failure of the endocrine therapy. The present report highlights Dual Specificity Tyrosine Phosphorylation Regulated Kinase 2 (DYRK2) as a predictive marker for everolimus sensitivity. The key node and KEGG pathway analyses revealed that mTORC1 pathway is activated in DYRK2-depleted cells. Everolimus was more effective in DYRK2-depleted cells compared with control cells. In xenograft model, everolimus treatment significantly inhibited tumor growth compared with vehicle or eribulin treatment. In clinical analysis, patients with low DYRK2 expression acquired longer treatment period and had higher clinical benefit rate than those with high DYRK2 expression (171 vs 82 days; P < 0.05 and 50% vs 12.5%, respectively). We further investigated the underlying mechanism by which DYRK2 regulates mTORC1 pathway. The ectopic expression of DYRK2 promoted phosphorylation of Thr631 for the ubiquitination and degradation of mTOR. DYRK2 expression levels may thus predict clinical responses to everolimus.